Explain [Analyze] Select Into table..... produces the plan which uses parallel scans.
create table table1 (n int);
insert into table1 values (generate_series(1,5000000));
analyze table1;
set parallel_tuple_cost=0;
set max_parallel_degree=3;
postgres=# explain select into table2 from table1;
QUERY PLAN
-------------------------------------------------------------------------------
Gather (cost=1000.00..39253.03 rows=5000000 width=0)
Number of Workers: 3
-> Parallel Seq Scan on table1 (cost=0.00..38253.03 rows=1612903 width=0)
(3 rows)
-----------------------------
So Explain Analyze Fails.
postgres=# explain analyze select into table2 from table1;
ERROR: cannot insert tuples during a parallel operation
STATEMENT: explain analyze select into table2 from table1;
But actual execution is successful.
postgres=# select into table2 from table1;
SELECT 5000000
Reason is in ExplainOneQuery we unconditionally
pass CURSOR_OPT_PARALLEL_OK to pg_plan_query even if query might be from
CreateTableAs/ SelectInto. Whereas in ExecCreateTableAs it is always 0.
Possible Fix:
I tried to make a patch to fix this. Now in ExplainOneQuery if into clause is
defined then parallel plans are disabled as similar to their execution.